Copper (Cu) is widely recognized as attractive to replace aluminum (Al) for interconnect conductors in next generation integrated circuits (IC's). Because of its lower resistivity, narrower copper conductors allow faster, higher density IC's than do aluminum conductors. Moreover, lower resistivity means lower energy consumption and heat dissipation. Copper shows much better electromigration resistance. This property provides for reduction of device operating voltage and better reliability. A problem with Cu interconnects, however, is that Cu diffuses into silicon (Si) at the contact regions as well as into intermetal dielectrics, causing device leakage and failure in manufacturing or operation.
Successful implementation of Cu as the primary conductor in the next generations of integrated circuits requires encapsulation of the Cu interconnection into diffusion barriers to block Cu migration. These barriers must possess superior metallurgical stability and electrical reliability, since barrier failure during device manufacture or actual operation would result in device leakage or breakdown. Conventional diffusion barrier materials include polycrystalline materials, such as titanium nitride (TiN), titanium tungsten (TiW), tantalum (Ta), and tantalum nitride (TaN). Diffusion of Cu along barrier grain boundaries has been identified as the primary failure mechanism with these materials.
Besides metallurgical and electrical reliability, Cu diffusion barriers have to meet other requirements, among them low electrical resistivity (typically &lt;500 .mu.ohm cm), good adhesion to dielectrics and metals, and chemical-mechanical polishing (CMP) compatibility. Good adhesion and CMP compatibility are difficult criteria to achieve with conventional barrier materials.
A variety of amorphous materials have been proposed as Cu diffusion barriers. For example, binary nitrides, e.g. TaN, TiN, WN.sub.x (tungsten nitride) and ternary silicon nitrides, e.g. TiSi.sub.x N.sub.y (titanium silicon nitride), WN.sub.x Si.sub.y (tungsten silicon nitride), and TaSi.sub.x N.sub.y (tantalum silicon nitride) have been identified as excellent Cu diffusion barrier materials. However, application of these materials is limited because of poor adhesion, high resistivity, and unknown chemical-mechanical polishing (CMP) characteristics.
Even the implementation of conventional Al contact plugs requires a diffusion barrier to prevent Al permeation into the device junction during high temperature Al fill (450.degree. C. to 500.degree. C.). Conventional TiN barriers have to be saturated with oxygen to prevent Al permeation. However, oxidation of the TiN surface makes the Al fill difficult due to a poor surface wetability.
A number of investigators have described the performance of refractory metals, nitrides, and related ternary compounds as copper and aluminum barriers for integrated circuit applications. Improvement of Cu diffusion barrier performance of W by surface nitridation has been described by Ono et al. (Jpn. J. Appl. Phys. 34, 1827 (1995)). Stress relaxation in a WNx/W bilayer has been described by Lee and Kim (Appl. Phys. Lett. 65, 965 (1994)). Silane treatment of metallorganic chemical vapor deposition (MOCVD) TiN films has been described by J. P. Lu et al. (Advanced Metallization and Interconnect Systems For VLSI Applications; October 1996, Boston; Proceedings pg. 45). In a different field of technology, multilayer metal/metal nitride films as wear-resistant coatings for cutting tools have been described by Shih and Dove (Appl. Phys. Lett. 61, 654 (1992)).
Needed in the art are diffusion barriers for applications in integrated circuits, specifically in sub-0.25 .mu.m logic, memory and application specific circuits with Cu as the primary interconnect conductor. These diffusion barriers must combine high overall electrical conductivity with reliable prevention of migration of Cu from interconnects into the semiconductor device. Additionally, the diffusion barriers must adhere well to the underlying materials as well as to Cu, and must exhibit mechanical integrity for chemical-mechanical polishing (CMP) compatibility.